Cell lines derived from a human ovarian cancer, which is sensitive to taxol, and TR, which is highly resistant, were clonally isolated. The latter clones were found to express P-glycoprotein (P-GP) by fluorescent staining with anti-P-GP monoclonal antibody and Western blotting. Transfection of the Multi-drug resistance type 1 (MDR-1) gene into TAOV cells (designated as 5E1) resulted in moderate resistance to taxol and expression of P-glycoprotein (P-GP), but the resistance was not dominant as in TR sub-clones. The influx of 3H-taxol into TR cells was 37-fold lower than that of TAOV cells, and 16-fold lower than that of 5E1 cells. Treatment of TAOV cells resulted in chromatin condensation and weak sign of internucleosomal DNA fragmentation. TR and 5E1 cells were resistant to the taxol-induced apoptosis. High performance liquid chromatography analysis of taxol metabolites in the taxol treated cells, and their culture media indicated that taxol was converted to one or two major metabolites in TR cells and pumped out efficiently in the media, contributing to minimize the taxol toxicity. Some of the single cell clones isolated from TR cells were found to proliferate markedly faster than the TAOV and 5E1 cells. These fast growing TR clones were insensitive to 24 hour treatment with 8-azaguanine (8-AZ), whereas 8-AZ killed all of the TAOV and 5E1 cells. About 50% of the 8-AZ resistant clones were resistant to a combined treatment of azaserine/hypoxanthine, suggesting that these fast growing taxol-resistant cells are not depending on the de novo passways for nucleotide biosynthesis. Amplified hypoxanthine-guanine phosphoribosyltransrase (HPRT) gene observed in the fast growing cells supported this interpretation. Although true mechanisms involved in the clonal development of the fast growing cells is not clear, unknown inheritable change(s) in the down-regulatory components of cell cycles during long term treatment with taxol may be a causative consequence that may have occurred in the taxol resistant cells. Further inter-relationship of drug resistance, cell cycle regulation, apoptosis and metabolic conversion in response to taxol treatment is a subject of future study.